Acylated derivates of polysaccharides, in particular starch, are important materials since acylation introduces useful functionalities, such as decreased or increased hydrophilicity, ionic functions, coupling functions, crosslinking, etc. Acylation is conventionally performed by treating the polysaccharide with an acylating agent such as an acyl halide or an acid anhydride. These acylating agents are typically more expensive and less environment-friendly than the acids themselves, and, moreover, these acylations result in an equivalent amount of a salt, according to the following reaction:Ps-OH+Ac-X+MOH→Ps-OAc+MX+H2O                (Ps=polysaccharide residue; Ac=Acyl group; X=anionic group such as chloride, bromide, acetate; M=metal such as sodium).        
Therefore, there is a need for more cost-effective methods of acylating starch and other polysaccharides, which do not produce substantial amounts of undesired by-products such as salts.
GB 815,358 discloses a process for treating starch with various etherifying or esterifying agents under dry conditions using e.g. nitrogen or superheated steam for fluidising the starch. The esterifying agents are anhydrides and acid chlorides, which still require the presence of neutralising agents.
U.S. Pat. Nos. 6,231,675 and 6,451,121 disclose thermally inhibited starches which are prepared by dehydrating a granular starch, using an alcohol, at a pH of about 9.5, and heat treating the dehydrated starch up to 20 h at 120-180° C.
Wing (Starch 48 (1996) 275-279) describes the production starch citrate by heating starch with citric acid at 110-140° C. for 24 h. Best results were obtained at low pH using phosphate catalysis.
WO 2004/104048 discloses a process for esterifying starch with acidified amino acids such as betaine and carnitine under dry conditions in an oven at 140° C. for 24 h. The products are water-soluble, which indicates that the starch is degraded during the process.